The presence of thyroid stem/progenitor cells has been demonstrated in recent years, however their specific cell surface markers are yet to be identified. We previously isolated Hoechst dye-resistant side population (SP) cells from mouse thyroids by fluorescence-activated cell sorting (FACS). SP cells are known to possess stem/progenitor characteristics. In fact, thyroid SP cells demonstrated stem/progenitor-like characteristics in terms of gene expression patterns and cell culture studies. To further characterize thyroid SP cells, microarray analysis was carried out using RNAs isolated from SP and non-SP cells of mouse thyroids, and the up- and down-regulated genes in thyroid SP cells were compared with those in non-SP cells. Gene expression analysis revealed that genes known to be highly expressed in cancer cells and/or involved in cancer invasion/metastasis were markedly up-regulated in SP cells as compared with non-SP cells. Among genes up-regulated, stanniocalcin (STC) 1 was chosen for further study. STC1 and 2 are secreted glycoproteins known to regulate serum calcium and phosphate homeostasis. In recent years, the relationship of STC1/2 expression to caner has been described in various tissues. The expression of STC1 was found in five human thyroid carcinoma-derived cell lines as revealed by analysis of mRNA and protein, and its expression was inversely correlated with the differentiation status of the cells. Immunohistochemical analysis demonstrated higher expression of STC1 in the thyroid tumor cell line and thyroid tumor tissues from humans and mice. These results suggest that SP cells contain a population of cells that express genes also highly expressed in cancer cells including STC1, which warrants further study on the role of SP cells and/or STC1 expression in thyroid cancer. We also used mouse as the model animal and partial thyroidectomy as a tool to activate stem/progenitor cells in the thyroid. This was based on the hypothesis that a sudden loss of thyroid tissue may activate otherwise dormant stem/progenitor cells to participate in thyroid regeneration. Beta-galactosidase reporter mouse was subjected to partial thyroidectomy in conjunction with bromodeoxyuridine (BrdU) long label-retaining cell analysis. Beta-galactosidase reporter mouse expresses beta-galactosidase only in differentiated mature thyroid follicular cells, thus allowing tracing the thyroid lineage by examining the expression of beta-galactosidase. In BrdU long label-retaining analysis, BrdU positivity was used as a surrogate marker for stem/progenitor cells due to the fact that BrdU positive stem/progenitor cells do not divide frequently or asymmetrically divide, thus BrdU being retained only in stem/progenitor cells. Based on the results obtained using these mouse models, we proposed two models for thyroid regeneration. Model I: Stem cell antigen (SCA)1(+) and newly synthesized BrdU(+) non-follicular mesenchymal cells appear after partial thyroidectomy, followed by appearance of SCA1(+);BrdU(+) intrafollicular cells, which eventually become part of functional follicles. Model I is operative in a situation where thyroid damage caused by partial thyroidectomy is moderate, such as removing the caudal one-third of both thyroid lobes, and takes 2-4 weeks to produce new intrafollicular cells. Model II kicks in when the damage is massive, exceeds the capacity repaired by Model I, and requires immediate repair to maintain body homeostasis and correct goitrogenesis condition. This is the case when the thyroid underwent semi-total partial thyroidectomy (one lobe and 2/5 of the other thyroid lobe removed), in which previously differentiated follicular cells and C cells appear to become immature cells and start contributing to thyroid regeneration. This model takes place within 1-2 weeks. Perhaps resetting many cells to the immature stage may make the differentiation/maturation proceed more robustly, resulting in more rapid restoration of thyroid function. The two thyroid regeneration models may be operable in any other damaged situations of thyroid such as radiation and chemical exposure, and may take place individually or simultaneously, depending on the extent of injury and regeneration required. These studies further suggest Sca1 as a possible marker for thyroid stem/progenitor cells.